1. Field of the Invention
The invention relates to probes for use in monitoring flow rate in fluids. The invention particularly, though not exclusively, relates to such probes used for monitoring the flow rate of natural gas in distribution pipes.
2. Background of the Related Art
A well-known kind of probe is the so-called Pitot-static tube. Such a tube is L-shaped and has a lower end portion extending parallel to the direction of the monitored flow. The tip of the end portion is presented towards the oncoming flow and has a first sensing port the plane of which is transverse to the monitored flow. A second sensing port, or each one of an array of second ports, is provided downstream of the first port in the wall of the end portion. The plane of the, or of each, second port is parallel to that monitored flow. The difference between the pressures at the first and second ports is proportional to the square of the velocity of the fluid at the tip.
The L-shape of such a probe precludes its general use because, for practical purposes, it is required that the probe be straight so that it can be inserted on withdrawn through a gland or valve in the wall of the pipe confining the monitored flow. A probe of straight construction has been proposed in the UK patent specification No. 2012056A (Dieterich Standard Corporation). In that proposal, an elongate body has longitudinally extending edges at its lateral surfaces. At each edge a boundary layer separates from the body. A sensing port, facing downstream, is positioned at the downstream side of the body in the wake of the two streams of fluid flowing around the body past the two edges. Thus, the port is located somewhat remotely from the two edges. The edges are required to be accurately and symmetrically formed on the body.